A known electronic key system allows a vehicle door to be unlocked when a user carrying a portable device (electronic key) approaches a vehicle door and allows an engine to start when the user enters the passenger compartment (refer to, for example, patent document 1). In the electronic key system, a communication area is formed around the vehicle door and in the passenger compartment. When the user who carries the portable device enters the communication area, wireless communication is performed between the portable device and an in-vehicle device installed in the vehicle. The wireless communication unlocks the vehicle door and allows the engine to start.
Except when the vehicle is driven, the portable device of the electronic key system is often left unused, for example, on a desk in a house. In such a portable device, the constant supply of power to the incorporated electronic component even when the portable device is not used consumes the power of the battery. Thus, power drainage of the battery is apt to occur.
A power saving mode may be set to reduce current in a standby state, and a particular switch operation may be performed to allow the portable device to shift to the power saving mode.